One of Facebook's most important sources of revenue is exploding

When app developers want to advertise to gain new users, they usually turn to Facebook.

In fact, Facebook is the "largest platform for app marketing," according to a new analyst note issued by Citi on Thursday.

Here's one statistic that drives that home: Citi estimates that 1.15 billion apps will be downloaded through marketing on Facebook properties in 2016. That's up 33% from last year, when 867 million apps were downloaded through Facebook, Facebook Messenger, Instagram, and WhatsApp, according to Citi's research.

Here are some more nuggets that showcase Facebook's success with app marketing:

The average price of an app install from Facebook is $3.40.

The average smartphone user downloaded .35 apps through Facebook in 2015.

Mobile-app-install advertising accounted for 17% of Facebook's total ad revenue in 2015, or $2.9 billion. (For context, total US mobile-app-install ad revenue was projected to top $4.6 billion in 2015.)

Facebook will be responsible for over 4 billion cumulative downloads by 2017, according to Citi's projections. ( Google announced in May that it has driven 2 billion cumulative app downloads to date.)

App-install ads are a lucrative niche for Facebook, but Citi's projections could be overly optimistic. One reason Facebook is getting so much app-marketing money is the main app stores run by Google and Apple aren't great at allowing consumers to discover new apps. But both companies have made changes to their app stores recently, and Apple recently introduced app-install advertisements of its own.

Also, the general slowdown of the smartphone market could mean there are fewer first-time smartphone users to throw ads at.

Regardless, here's how Citi thinks Facebook's app-install business will grow: